I am using LabView 6.1, NI-SCOPE software, and an NI 5102 digitizer.

[jrs100]

I am using both channel 0 and channel 1 and need to plot voltage versus voltage. Can someone help?

Thanks,
-Robby

[DFGray]

Use any of the multi-channel acquisition examples, then connect the output data to an XY Graph (select Graph->XY Graph from the palette). Or save the example as a different VI and replace the Waveform Graph on the front panel with an XY Graph.

[jrs100]

Thank you for your help. I found a multi-channel acquisition vi (attached) from the NI web site but am not able to connect to an XY graph when I change from the waveform graph. Is there a compatibility problem with the multi-read icon and the XY graph?

Thanks,
-Robby

[DFGray]

Yes, there is. The XY graph needs a cluster of two arrays or an array of XY data pair clusters. I modified your VI and attached it below. Unfortunately, this makes a couple of copies of data. You will want to use the lower level fetch routines if you are fetching data sets over about 1MByte in size.

I saved the VI as LabVIEW 6.1, but am unable to test, as the latest version of NI-SCOPE does not support LabVIEW 6.1.

[jrs100]

Thank you for your help. To test the file, I used a function generator card and another data acquisition card with analog out to send sine wave voltages to CH0 and CH1 and plot the two signals against one another. The vi worked perfectly. Thanks!

I'm onto the next step of my problem which is to take an amplified signal from two differential pressure transducers and plot them versus each other. I've read unexpectedly in the NI 5102 users manual that "you cannot use CH0 or CH1 to make differential measurements or measure floating signals unless you subtract the digital waveforms in software." (page 4-6 June 2001 Edition). I've also seen on-line (NI site) that you can't use CH0 or CH1 to make differential measurements unless you subtract as above OR use costly probes. I assume the differential pressure transducers that we have would work for that purpose. I've tested the amplified signal with a voltmeter and get a responsive signal, but when I try to see the input using NI-SCOPE and a front panel or the vi that was developed I don't get a signal or the input signal I see appears but then disappears to not reappear.

Based on this, do you have any way of knowing if I have a signal incompatibility issue?

-Robby

[DFGray]

It sounds like you have some problem, but not necessarily the one you think. The differential in the name of your sensor refers to the pressure, not the voltage/current output (although the output may also be differential, without a spec sheet, I don't know). Did you check to make sure the gain on the 5102 was set all the way up? Most pressure sensors have a pretty low output range. If the output range is low enough, you may be getting noise on the low bits of the 5102 output. It is only an 8 bit device (256 levels). If this is the case, you have several options:

1. Find an amplifier(s) and use it(them)
2. Roll your own amplifier. This is much easier than you may think. A $2 amp and $30 of supporting circuitry and packaging will do what you want. Go to the National Semiconductor or TI websites for chip selection and suggested circuit diagrams. Use a wall-plug transformer or batteries for power.
3. Get a new sensor which has a higher output. This may be cheaper and easier than making your own amplifier.
4. Get a data acquisition board with better gain and/or resolution. This is more expensive. Currently, low-cost USB solutions start at about $150, plug-in boards at about $375. If you don't need speed, go USB. It is much cheaper and you don't need special cables and connectors to plug into it.

Good luck!

[jrs100]

I am using a carrier demodulator with the differential transducers to amplify the signal, and get a 1-2 V reading (on a voltmeter) for the amplified signal. It's just when I plug the output into the digitizer that I have problems. I have not checked the gain on the digitizer, but will do that and will also look further into the options that you presented.

Thanks,
-Robby